Don't Forget to Reset the SMC
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My girlfriend's 2009 unibody MBP was seeing some pretty poor battery life. The nearest Apple-certified shop seemed keen to charge extra for installation. I'd just upgraded the HDD, so why not do the battery as well?
I ordered a new battery from iFixit. Installation was quick & painless, ~15 mins tops. Luckily I got one of the iFixit kits randomly from ThinkGeek, or those tri-screws at the battery would have been a nightmare.
Reset the SMC after installing the new battery! The laptop adamantly believed the battery was old and died quickly, until I reset the SMC and all was well (see here for the steps: http://support.apple.com/kb/HT3964 ).
This step is not listed in the Battery Replacement Guide, but is necessary to use a new battery to its fullest potential.